Process for creating a durable EMI/RFI shield between two or more structural surfaces and shield formed therefrom

ABSTRACT

A method for producing a durable electromagnetic and radio frequency interference shield between two or more structural members of a wall or enclosure and a shielded shelter produced using the method. In one embodiment, joints between structural members are preferably filled with an electrically conductive filler. A base coat of a metal spray that adheres well to the filler and structural member is then applied. At least one layer of a metal spray with magnetic field attenuation properties such as steel, and at least one layer of a metal spray that has plane wave attenuation properties such as tin are applied to the base coat. Optionally, a coat of protective or conductive paint is then applied to the top surface of the metal spray layers. An enclosure with shielded joints according to the present invention has superior shielding capability and durability over the art.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/446,442, filed on Jun. 1, 2006, incorporated herein by reference inits entirety, now U.S. Pat. No. 7,307,223, which is a divisionalapplication of U.S. application Ser. No. 10/360,528, filed on Feb. 7,2003, now U.S. Pat. No. 7,063,767, incorporated herein by reference inits entirety, which claims priority from U.S. provisional applicationSer. No. 60/355,130, filed on Feb. 8, 2002, incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to EMI/RFI penetration resistantstructural materials, and more particularly to a process for creating adurable EMI/RFI shield between two or more adjacent metal surfaces.

2. Description of Related Art

Many household, industrial, commercial and military electronic devicesare susceptible to interference by electromagnetic and radio frequencyradiation sources in their surrounding environment. A radio frequencycircuit, for example, often requires shielding to preventelectromagnetic energy generated by the circuit from interfering withthe circuit or with the operation of adjacent electronic devices.Likewise, in military applications, sophisticated communications,command and logistical equipment are vulnerable to electromagneticinterference in battlefield settings. Both enemy and friendly sources ofelectromagnetic interference may disrupt sensitive equipment renderingit temporarily or permanently inoperative. Natural or man-madeelectromagnetic pulses may also significantly disrupt equipment.

Additionally, electromagnetic sources created by certain equipment mayalso be intercepted and used by an enemy to locate the radiation source.Similarly, transmitted communications and unencrypted data that isradiating within an enclosure or room may be intercepted by an enemy ora corporate spy allowing a loss of private or essential information.Therefore, it is essential that any enclosure or room that housessensitive equipment or systems provide a shield to protect the equipmentfrom damage or thwart the compromise and loss of vital information.

Electromagnetic interference (EMI) can occur at frequencies anywherealong the electromagnetic spectrum. The radio frequency part of theelectromagnetic spectrum is normally considered to include the range ofbetween 10 kilohertz (KHz) and 10 gigahertz (GHz) and is included in theterm electromagnetic radiation. Radio frequency interference (RFI) andelectromagnetic interference (EFI) are use interchangeably herein.

A wire or a trace on a printed circuit board, for example, can act as anemitter of electromagnetic interference by the action of moving acurrent through the wire that creates an electromagnetic field. A wireor trace may also act as a receptor of electromagnetic interference byexposure to an electromagnetic field. Consequently, the enclosures ofsensitive electronic devices preferably shield the circuitry fromoutside electromagnetic interference (EMI) by either reflecting orabsorbing the electromagnetic energy so that the energy in theenvironment surrounding the electronic devices remains at acceptablelevels.

One generally known EMI/RFI shield is a housing that is electricallyconductive and is electrically grounded. Current is caused to flow in aconductive barrier when exposed to an electromagnetic force field. Whenthe electromagnetic field penetrates the conductive barrier, the currentis attenuated e.g. reduced in amplitude in what is known as the skineffect. The current flow in the barrier is approximately equal to twicethe magnetic field strength incident to the barrier when the field isperpendicular to the barrier and is called the surface current density.Surface current density is typically measured in amps/meter. It has alsobeen shown that the power of the electromagnetic field as it leaves theconductive barrier is approximately equal to the impedance of thebarrier times the square of the current. The power is usually determinedin watts per meter squared. Accordingly, electromagnetic radiationemanating from the inside or the outside of the housing is absorbed bythe conductive material and dissipated through the ground and away fromthe sensitive electronic components thereby permitting the properoperation of the electronic equipment.

However, one deficiency with such shields is that the electricalcurrents created in the conductive material of the housing can bedisrupted by gaps between panels or around access doors and the like andcan hinder the conduction of the EMI energy to the ground. Such gaps maylead to leakage EMI energy through the shield. In some circumstances,the gaps may act like slot antennas resulting in the shield becoming asecondary EMI source. This is due to the fact that a voltage is createdacross the seam that is approximately equal to the current times theimpedance of the seam. Reducing the impedance of the seam through theuse of gaskets or the like may reduce the power radiating from the seam.

Shelters and other enclosures of electronic equipment have also beendeveloped to provide EMI shielding for military and civilian electronicequipment. For example, an EMI shielded shelter is described in U.S.Pat. No. 6,111,192 that is collapsible for easy transport. Such sheltersare typically designed to fit on the back of a truck for mobility.However, such shelters may also experience electromagnetic leakagethrough the joints between panels, doors, hatches and other access ways.The overall capacity of the shelter to shield electromagneticinterference can be reduced by leakage through seams between panels.

Another deficiency in the EMI shields known in the art is that theoverall shielding capacity of the shelter degrades over time. Thisdegradation is particularly apparent with shelters that are regularlytransported or exposed to severe weather conditions and temperaturefluctuations.

Therefore, there is a need for an EMI/RFI shield or shelter thatefficiently protects sensitive electronics from interference andeffectively eliminates leakage between the panels or sections of theshield. The present invention satisfies that need, as well as others,and overcomes deficiencies found in prior methods and structures.

BRIEF SUMMARY OF THE INVENTION

The present invention generally comprises an EMI/RFI shield includingtwo or more adjacent structural surfaces as well as a method forproducing the shield. It can been seen that seams and joints in anEMI/RFI shelter, for example, may limit the shielding capability of theshelter when exposed to a radiated electromagnetic field.

Seams may allow the electromagnetic interference fields to bypass or betransmitted through the shield thereby limiting the overall capabilityand effectiveness of the shield. The impedance of the seam or joint musttherefore be minimized, preferably near zero.

By way of example, and not of limitation, in accordance with one aspectof the invention an enclosure is provided that has base structural wallswith joints that are preferably filled with conductive filler. Thestructural members of the enclosure preferably have applied a bondinglayer, at least one layer with magnetic field attenuation properties andat least one layer with plane wave attenuation properties.

According to another aspect of the invention, the seams between adjacentstructural members are bridged with the bonding, magnetic field andplane wave attenuation layers thereby providing a shield even if thoselayers later separate from the surface of the seam or filler.

Generally, the preferred method of the invention for providing anEMI/RFI shield comprises providing a base structure and applying a bondcoat of a conductive metal spray to the base structure. A coat of asecond conductive metal spray is then applied to the bond coat and thena coat of a third conductive metal spray is applied to the second coat.The layers of metal spray may be applied to the interior of theenclosure, the exterior of the enclosure, or a combination of theinterior and exterior of the enclosure.

The preferred base structure is aluminum or aluminum alloy sheets foruse in transportable structures. The sheets may also be sandwich sheetswith a honeycomb or foam core. While aluminum is preferred for weight,corrosion and cost considerations, other metals, alloys and non-metalcomposite materials may also be used.

By way of example and not of limitation, the bond coating step iscarried out by applying a base or bond coat of a preferably conductivemetal spray. The preferred bond coat comprises a coat of molybdenum orother metal that provides a good tensile bond with aluminum basematerial. The bond coat is preferably approximately 2 millimeters at aminimum.

The second layer is preferably a metal spray of a second metal appliedto the bond coat in one embodiment. The second metal spray is preferablysteel or a steel alloy. The steel spray is preferably betweenapproximately 2 and 10 millimeters thick. While steel is preferred othermetals with magnetic field attenuation properties may also be used.

The outer layer is preferably a third metal applied by spraying over thesecond metal layer. The preferred outer layer is tin or 80%/20% tin/zincalloy. Tin or a tin alloy is preferred because it provides plane waveattenuation properties and a protective coat to the under layers. Whilethe sequence of steel and tin or tin/zinc spray applications ispreferred, it will be understood that any sequence magnetic field andplane wave attenuating spray layers may be employed. Furthermore, thebase structural member, for example, may also separate the layers by onelayer being applied to the interior of the structure and one layer beingapplied to the exterior.

In another embodiment, seams and imperfections are filled with aconductive filler such as steel powder impregnated epoxy. The bond coatis then sprayed over the filler. If the coverage of the bond coat issatisfactory, then at least one coat of a second metal is applied to thebond coat and a coat of a third metal is applied to the second coat asdescribed previously.

If the bond coat is unsatisfactory due to poor adhesion or coverage, anintermediate layer of a metal is applied to the bond layer in thisembodiment. The intermediate layer is preferably a coat of tin or tinalloy metal spray. While tin is preferred, other metal sprays includinganother coat of the bond metal spray may be used. The steel layer andouter tin layer are applied to the intermediate layer in thisembodiment.

In another embodiment, the outer metal layer receives an optional outercoat of a urethane; elastomeric coating or paint to provide someprotection from scrapes and impacts. In another embodiment, the paint isa conductive paint to further enhance the EMI/RFI shielding capabilityof the shelter.

Therefore, according to an aspect of the invention an EMI/RFI shield isformed between two or more adjacent metal surfaces by bridging theadjacent metal surfaces using a metal spray material. Another aspect ofthe invention is to minimize flexure in the bridge of the metal spraymaterial. A still further aspect of the invention is to clean theadjacent metal surfaces prior to applying the metal spray material.Another aspect of the invention is to fill any irregularities betweenadjacent metal surfaces with an adhesive material having conductiveproperties after the cleaning step.

An object of the invention is to provide an electrically conductiveEMI/RFI shielding between two or more adjacent metal surfaces that isdurable and has low resistance.

Another object of the invention is to provide a method for forming anEMI/RFI shield on a housing or shelter of electronic equipment that canbe applied to metal or non-metal substrates.

Still another object of the invention is to provide a shelter that iseffectively resistant to the penetration of electromagnetic interferencethat is has a shield that is resistant to fatigue, chipping andcracking.

A further object of the invention is to provide a shelter that isreadily transportable and resistant to a wide range of electromagneticinterferences.

According to another object of the invention, an EMI/RFI shield isprovided that is durable and resistant to shock, vibrations and severeweather conditions and still maintain performance and has a corrosionresistant outer coating.

Further objects and advantages of the invention will be brought out inthe following portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings that are for illustrative purposes only:

FIG. 1 is a flow diagram showing generally the method steps of oneembodiment of a method of providing a durable EMI/RFI shield between twoor more surfaces according to the present invention.

FIG. 2A and FIG. 2B is a flow diagram showing generally the method stepsof an alternative embodiment of a method of providing a durable EMI/RFIshield between two or more structural surfaces according to the presentinvention.

FIG. 3 is a cross sectional view of one embodiment of a durable EMI/RFIshield between two or more structural surfaces according to the presentinvention.

FIG. 4 is a cross sectional view of an alternative embodiment of adurable EMI/RFI shield according to the present invention.

FIG. 5 is a perspective view of one embodiment of an EMI/RFI shelteraccording to the present invention adapted to be transported by avehicle.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the method and apparatus generallydepicted in FIG. 1 through FIG. 5. It will be appreciated that theapparatus may vary as to configuration and as to details of the parts,and that the method may vary as to the specific steps and sequence,without departing from the basic concepts as disclosed herein.

Turning now to FIG. 1, a flowchart of the preferred embodiment of themethod 10 for providing a durable EMI/RFI shield between two adjacentstructural surfaces is shown. The structural materials that areparticularly suitable for use with the method are materials that areshown to exhibit resistance to electromagnetic interference penetration.Typically, metal or metal alloy sheets of varying cross sectionalthickness may be used. However, sheets of non-metal composite materialmay also be used with the method. The materials may be single sheets,two or more sheets and a frame/core structure forming sandwich panels,or two or more sheets and a frame structure forming panels.Additionally, any cutout or penetration made to the enclosure to allowsignal, power, or air can utilize this technique between the sheet skinand the corresponding cutout panel.

At block 12, the surfaces of the two panels or other structural membersare prepared. The surfaces are prepared by removing any paints, primers,adhesives or other foreign materials from the surfaces of the panels. Inaddition, any oxidation or other metal reactant products andcontaminants should be removed from the surfaces. The surfaces may beprepared, for example, by sanding, sandblasting, grinding, wire brushingor exposing the surfaces to solvents to remove the contaminants. It willbe understood that the preparation step may be a single step or a seriesof steps depending on the nature of the surface contaminants andcomposition of the panel.

The edges of the joints between two or more adjacent surfaces arepreferably machined so that the joints are precise. The joints of panelsthat are side by side are preferably sanded and planed so that panelsare aligned and generally in the same plane. Although tight joints arepreferred, it will be seen that imprecise joints can also be bridgedusing the process.

Sanding and solvent residue is preferably removed from the preparedsurfaces by mechanical means such as forced air, vacuum or cloth andthen thoroughly cleaned with isopropyl alcohol or other suitable solventknown in the art that will not react with the surface panel or appliedmaterials or leave an appreciable residue.

Any irregularities of the prepared surfaces and the joints are thenfilled with a filler at block 14. In the preferred embodiment, thefiller is electrically conductive and applied soon after the surface isprepared at block 12 so that oxidation or other reactions of theprepared metal surface will not occur.

The preferred filler is a particulate steel or other metal impregnatedepoxy. However, other conductive or non-conductive fillers may be used.It is preferred that the filler be thermally stable and capable ofexposure of up to approximately 400° F. The preferred filler should alsobe galvanically compatible with the structural members or the base coatof metal spray that is applied in step 16. Optionally, it is alsopreferred that the filler have a reasonable curing time of less thanapproximately twelve hours and a set up time of less than approximatelytwo hours as well as being able to be applied in temperatures rangingfrom approximately 40° F. to approximately 100° F. The selected fillershould also provide a suitable bonding surface for subsequentapplications of spray.

After the filler has cured or dried, the filled areas are preferablysanded so that the surface is even and the surface of the filler isflush with the adjacent metal surfaces as shown in FIG. 3 and FIG. 4. Inone embodiment, the filler is sanded to a slight taper to allow forslight misalignment of the surfaces of the structural members. Sandedsurfaces that do not have any shadows or pits or irregularities arepreferred so that an even bonding surface is provided for theapplication of the base metal spray at step 16.

The filled and sanded surface is further prepared by the removal of dustand other contaminants. Particle removal may be accomplished with theuse of forced air, vacuum or by wiping the surface with a cloth.Thereafter, the surface may be cleaned thoroughly with the applicationof isopropyl alcohol or other solvent that leaves no appreciable residueand will not react with the materials.

At block 16, a base coat of a metal spray is applied to the filled andprepared surfaces. The preferred minimum thickness of the base coat isbetween approximately 2 millimeters and approximately 3 millimeters. Thebase coat is preferably applied at block 16 shortly after the surface issanded and prepared to avoid oxidation of the surfaces before theapplication of the base coat. In one embodiment, the base coat is notrequired to adhere or completely adhere to the filler as long as thebase coat of metal spray adheres to the surfaces surrounding the filler.

The preferred base coat metal spray is 99% pure molybdenum for basesurface substrates made of aluminum or alloys of aluminum. Although puremolybdenum is preferred, it will be understood that alloys of molybdenumor other metals may be used as a base spray that is preferably selectedby the ability of the material to adhere and provide a good tensile bondto the structural panel or by the ability of the material to provide afoundation for a second metal spray selected for application at block18. For example, suitable alternative bond coat materials include, butare not limited to aluminum/nickel/molybdenum alloys and nickel/aluminumalloys. Aluminum/bronze and aluminum/bronze/nickel alloys are lesspreferred in applications that will be used in humid weather conditionsdue to the potential for corrosion.

Any dust or over-spray particles from the base coat of metal spray arepreferably removed with forced air or vacuum. Any contamination of thesprayed surface by oils or other contaminants from the hands orcompressed air for example is preferably avoided.

At block 18, a second metal spray coat is applied to the base coat ofmetal spray. The coat of a second metal spray is preferably appliedwithin approximately one hour of completion of the base coat to avoidexposing the surfaces to contaminants as well as denying the opportunityfor oxidation of the base coat.

Once the coat of a second metal spray is applied, any spray particlesthat have not adhered to the previous coat and other contaminants areremoved, preferably by forced air or vacuum. Liquid contaminants such asoil should also be avoided because these contaminants may interfere withthe adhesion of subsequent coats of metal spray. In one embodiment, thesecond metal spray layer comprises .80C steel because steel providesgood magnetic and some plane wave attenuation capability. While highcarbon steel is preferred, it will be understood that any iron or steelalloy with good magnetic impermeability properties will be a candidatematerial for a metal spray layer.

A third metal spray is applied at block 20 in the embodiment shown. Themetal used for the third metal spray is preferably a metal that isenvironmentally inert to avoid oxidation and provide protection to theunderlying coats as well as provide additional plane wave attenuation.In one embodiment, the selection of the metals of the base coat, thesecond metal coat and the third metal spray coat is made by optimizingthe thermal expansion, bond strength and magnetic and galvanic couplingproperties of each metal. Materials with closely matched thermalexpansion coefficients, for example, will avoid temperature type cyclingfatigues and the EMI/RFI attenuation capability of the panels and jointswill not degrade over time.

Optionally, at block 22, one or more coats of a conductive paint can beapplied to the surface of the panels and joints to enhance the overallEMI/RFI shielding performance of the panels. Alternatively, an outerprotective coat of non-conducting polyurethane with pigments may beapplied to the exterior of the panels or structural members. Top surfacecoatings preferably provide some protection to the layers of metal sprayfrom scuffs and other impacts that may compromise the integrity of themetal spray layers.

Turning now to FIG. 2A and FIG. 2B, there is shown a flow diagram thatgenerally outlines the steps of a second embodiment of the presentinvention. Referring also to FIG. 3 and FIG. 4, the second embodiment,adapted to metal panels such as those made of aluminum or aluminumalloys, is shown in cross-sectional views.

The metal panel or structural members 24 a, 24 b shown in FIG. 3 andFIG. 4 are typically sandwich panels of aluminum skins with anon-metallic honeycomb core. The core is usually composed of paper orNomex® honeycomb but may also be foam. The thickness of the sandwichpanel is preferably approximately 1.26 inches but may vary from 1 to 3inches in the typical shelter application. The skin thickness typicallyranges from approximately 0.016 inches to 0.063 inches with a thicknessof approximately 0.025-0.032 inches being preferred for use with ashelter, such as shown in FIG. 5.

With other applications, the metal panels 24 may be single sheets orsandwich panels of essentially any thickness. The metal panels 24 aretypically perimeter welded together to form the desired structure.However, other methods of bonding panels known in the art such asadhesives may be used as well.

At block 12 of FIG. 2A, the surfaces of the structural members isprepared to remove any excess adhesives, paints, primers, weldingresidues, oxidation and the like that may interfere with the adhesion ofa base coat of metal spray or fillers.

The surface is preferably cleaned with isopropyl alcohol to removesurface contaminants and particulate matter from the surface of themetal panels 24 a, 24 b.

A steel particulate impregnated epoxy conductive filler 26 is thenplaced into irregularities and joints of the panels or structuralmembers as shown at step 14 of FIG. 2A. While steel particulateimpregnated epoxy is preferred, other metal particles and otheradhesives and fillers may be used. Once the filler 26 has had sufficienttime to cure, the filled areas are preferably sanded or otherwisemachined to a level with the planes of the panels or structural members.Particulate matter is then removed from the surfaces with pressurizedair or vacuum and then the surfaces are cleaned with a solvent,preferably isopropyl alcohol, to remove any remaining contaminantsincluding oxidants.

At block 16 of FIG. 2A, a base coat of a metal spray 28 is applied tothe prepared surfaces of the panels 24 a, 24 b. The preferred metalspray of the base coat 28 is approximately 99% pure molybdenum, selectedbecause of its ability to form strong bonds with the aluminum panels andto the preferred metal impregnated filler material. Although molybdenumis preferred, other metals or metal alloys that can form good bonds withaluminum can be used as a base coat. For example,Nickel/Aluminum/Molybdenum and Nickel/Aluminum may be used. An evenlyapplied base coat of between approximately 2 millimeters andapproximately 3 millimeters is preferred. The base coat is then exposedto forced air or a vacuum to remove any particulate matter and theninspected.

At question block 30, the results of the inspection of the base coatdetermine whether the base coat is satisfactory. The base coat will beconsidered satisfactory if there is an even coat over the panels and themetal spray has properly adhered to the filler at the seams etc.Sections of filler between two metal surfaces that are large will oftenhave insufficient contacts with the base coat of metal spray and willcreate an area of weakness.

If the base coat is determined to be satisfactory at block 30, then asecond layer of metal spray 32 is applied at the step at block 34 ofFIG. 2B. FIG. 3 shows a cross section of the EMI/RFI shield if the basecoat is determined to be satisfactory at block 30 of FIG. 2A. The secondlayer of metal spray is preferably .80C steel evenly applied to athickness of approximately 2 millimeters to approximately 10millimeters. Steel layers of greater than approximately 10 millimetersare not preferred because of the propensity of the layer to peel,particularly when exposed to jolts or significant vibrations. While .80Csteel is preferred, it will be understood that other carbon contentsteel or iron sprays and the like may be used that have magneticattenuation properties. Additionally, .80C steel is also preferredbecause of its thermal expansion compatibility with mating materials,bond strength with the base coat and its magnetic and galvaniccompatibility with the mating materials in this embodiment.

The coat of a second metal spray 32 shown in FIG. 3 is preferablyapplied within approximately one hour of the application of the basecoat to avoid oxidation and contamination of the base coat. Afterapplication, any residual particulate matter on the second metal spraycoat 32 is preferably removed with forced air or a vacuum.

Referring now to block 36 of FIG. 2, an outer coat 38 of a third metalspray is applied to the cleaned steel coat 32 shown in FIG. 3. Thepreferred outer coat of metal spray 38 is 99% pure tin or an 80%/20%tin/zinc alloy. The outer coat 38 is preferably sprayed to a thicknessin the range of approximately 5 millimeters to approximately 80millimeters. Thicker outer coats 38 are preferred for applications thatintended for use as contact surfaces for removable panels, filters, etc.The additional thickness also allows for long term repetitive cleaningof the surface and protection against severe weather or other oxidationconditions. In addition, the outer coat of tin or tin alloy providesplane wave attenuation and thicker outer coats provide greaterattenuation capability to the structure. While tin or tin alloys providea durable protective coat as well as plane wave attenuation, it will beunderstood that other metals and alloys or materials that provide planewave attenuation, and durability if necessary, that can be used as anouter coat.

While tin or tin/zinc alloys are preferred for the outer coat in theembodiment shown, other metals and metal alloys may be used as well. Forexample nickel metal and nickel metal alloys such as nickel/chrome aswell as other tin alloys may be used.

If the base coat 28 is determined to be unsatisfactory upon inspectionat block 30 of FIG. 2A, an intermediate coat of a second metal spray 40is applied to the base coat 28 as shown at block 42 of FIG. 2B. Theembodiment shown at FIG. 4 is one that is produced when the base coat isdetermined to be unsatisfactory at block 30. The intermediate coat ofmetal spray 40 is preferably composed of either 99% tin or an 80%/20%tin/zinc alloy. The intermediate metal spray 40 is preferably evenlyapplied in a thickness of between approximately 2 millimeters and 3millimeters. Contaminants and particulate matter are then removed fromthe intermediate coat of spray 40 using preferably forced air or avacuum.

A coat of a third metal spray 44 is then applied at block 46 of FIG. 2Bto the intermediate coat of metal spray 40. In the embodiment shown inFIG. 4, the third metal spray is preferably .80C steel. The steel coat44 is preferably of a thickness within the range of approximately 2millimeters and approximately 10 millimeters evenly applied.

Layers greater that approximately 10 millimeters are not preferredbecause of the appearance of peeling in applications where the panelsare exposed to harsh or rugged conditions.

At block 48 of FIG. 2B, the steel coat 44 is then coated with an outercoat 50 of a metal spray. The preferred outer coat of metal spray is 99%pure tin or an 80%/20% tin/zinc alloy, which gives the best combinationof durability, corrosion protection and EMI/RFI shielding attributes.The preferred thickness of the outer coat of metal spray 50 is withinthe range of approximately 5 millimeters and approximately 80millimeters depending on the intended use and the performance needs ofthe shield.

Optionally, one or more coats of paint or sealants can be applied to theouter coat of metal spray 50 and surface panels and joints as providedat block 52 of FIG. 2B. Conductive paints may also enhance the EMI/RFIshielding performance of the system. Alternatively, an outer protectivecoat of non-conducting polyurethane or elastomeric coating may beapplied to the exterior of the panels or structural members. Top surfacecoatings preferably provide some protection to the layers of metal sprayfrom scuffs and other impacts that may compromise the integrity of themetal spray layers.

Additionally, conductive and non-conductive paints may fill any pores ordefects in the layers of metal spray and thereby restrict access of airand moisture to the lower spray levels and therefore the occurrence ofcorrosion and the like.

The applications of metal spray described in FIG. 1, FIG. 2A and FIG. 2Bmay be made to the interior or to the exterior surfaces of the enclosureor a combination of both. In one embodiment, bond coats, magneticattenuation coats and plane wave attenuation coats of metal sprays areapplied to both the interior and exterior surfaces of the structuralmembers.

Although single spray layers in a particular sequence are illustrated inFIG. 1 through FIG. 4, it will be understood that multiple spray layersof magnetic field attenuation metal sprays and plane wave attenuationmetal sprays may be applied and that the sequence of application ofthese sprays may be changed or alternated. For example, alternatinglayers of steel metal spray and tin metal spray may be used to form athicker laminate of sprays to increase the durability and attenuationcapability of the shield.

An example of an EMI/RFI shelter application configured for placement onthe back of a truck or trailer is shown in FIG. 5. The shelter shown inFIG. 5 has a generally rectangular shape with left side-wall 54, rightside-wall 56, bottom 58, top 60, front 62, and back 64. Indentedsections 66 and 68 are present to allow the structure to fit over wheelsor wheel wells on a truck or trailer. An access door 70 on the back wall64 provides entry into the shelter.

In the example shown in FIG. 5, the side walls 54, 56, front and backwalls 62, 64, top and bottom panels 60, 58 and door 70 are made from6061-T6 aluminum skins bonded with adhesives to a perimeter 6061-T6aluminum extrusion frame and a non-metallic honeycomb core having athickness of approximately 1.21 inches with the overall panel thicknessof approximately 1.26 inches. The panels are sized and welded togetherto make the shelter. The seams and any imperfections in the surfaces ofthe structural elements are filled with a metal impregnated epoxyfiller.

After allowing the filler to cure, the excess filler is removed and theedges are smoothed and shaped. The surfaces are cleaned and a bond coatof molybdenum is applied followed by a coat of steel spray and then acoat of tin spray as described in FIG. 1 and FIG. 2A and FIG. 2B. Allseams and permanent penetrations of the panels of the shelter arepreferably treated with the process.

The embodiment of the present invention shown in FIG. 1, FIG. 2 and FIG.5 will produce an EMI/RFI shield with the following typical minimumattenuation levels that will not substantially diminish over time:

156 KHz>60 db

16 MHz>90 db

400 MHz>93 db

1 GHz>85 db

10 GHz>85 db

Generally, during the metal spray steps of FIG. 1 and FIG. 2A and FIG.2B, it is preferred that a visual inspection of each spray layer beconducted to insure that the metal spray does not overheat the bondingsurfaces to the point of degrading the overall structure. Overheatingcould cause debonding of the sheet skin from the frame structure orhoneycomb core, resulting in a structural and/or EMI/RFI deficiency orweak point that could degrade over time. In addition, exceeding therecommended ranges of layer thickness may result in the introduction ofexcessive tensile or compressive stresses within the metalized surfaces.Such stresses may lead to fatigue and fractures in the layered surfacesduring use that may allow leaking of electromagnetic or radio frequencyradiation.

It can be seen that the selection of metal sprays and dimensions shownin the examples above was made to provide minimal flexure in the bridgesof metal spray in order to avoid fatigue stressing from road vibration,for example. Additionally, the materials that have been selected haveclosely matched thermal expansion coefficients to avoid temperaturecycling type fatigue fractures.

While the materials and dimensions discussed above were selected fordurability and performance in severe environmental conditions, othermaterials and dimensions with comparable characteristics may be used andthe invention should not be limited to those materials and dimensionsdescribed herein. Still other materials and dimensions may be selectedfor applications where temperature, shock, vibrations, weatherconditions are not of concern.

The present invention may be more particularly described in thefollowing example that are intended for illustrative purposes only,since numerous modifications, adaptations and variations will beapparent to those skilled in the art.

Example 1

A lightweight multipurpose shelter, configured for placement on the backor frame of a truck, was produced using aluminum skin, hot bonded panelswith a welded aluminum extrusion framework. The exterior dimensions ofthe shelter were 84 inches in width by 102 inches in length by 67 inchesin height.

Extruded aluminum frame members were sized, cut and then welded togetherto create panel frame weldments. A honeycomb core material was placedinto the frame weldments and properly dimensioned panels of aluminumsheets were sized, cut and hot bonded to both sides of the frameweldment/honeycomb core structure using film adhesive and core spliceadhesive materials. The surfaces of the top, bottom, sides, front andback were prepared by removing all adhesive and core splice residue, aswell as cleaning off the corrosion inhibiting primer from extruded framemembers and the interior skin surface of the panel approximately ½″ awayfrom the bond line between the frame member and the interior skin. Carewas taken to only remove a minimal amount of frame and panel interiorskin material during this process. Any film adhesive and/or core splicematerial was cleaned out of any gaps between frame member and theinterior skin by wire brushing gap until bare metal was exposed. Thebrushed surfaces were prepared by cleaning with isopropyl alcohol.Likewise, the interior doorframe perimeter where the skin meets thedoorframe extrusion was wire brushed to remove all excess film adhesivein this area and the brushed surfaces were cleaned with alcohol.

Epoxy was injected into the seam gaps around the entire interiorperimeter of the wall, floor and top and front and back panel jointsinsuring that sufficient material was used to completely fill the gaps.Similarly, a bead of epoxy was placed along the prepared perimeter ofthe interior doorframe to bridge the skin and the frame member.

After the epoxy cure, the excess epoxy material was sanded down until itwas just flush with the panel's interior skin and frame surfaces for allof the seams. Corner fittings were installed at the bent wall panelcorners and trimmed as necessary for a snug fit.

The assembled structure is loosely strapped with metal strapping and thepanels were adjusted to insure the structure was square and had theproper dimensions. Overall tolerance dimensions were plus/minus ¼″. Thedimensions of shelter faces were recorded and the metal banding strapswere tightened in preparation for tack welding.

The dimensions were verified and the exterior seams of the corners ofthe front and rear walls of the shelter were tack welded into position.The overall dimensions were adjusted and evenly spaced tack welds of nomore than 18-inch separation between welds were performed along the seamperimeter of the shelter.

The entire exterior perimeter seams of the shelter were skip weldedfollowed by skip welding of the interior perimeter seams. Rectangularwheel wells were then positioned and skip welded into place to the walland floor panels. The corner fittings were then welded into place.

For all tack welding and subsequent skip welding, the surfacetemperature of the panel was monitored during welding to insure that thesurface temperature of panel interior and exterior skins never exceed225° F. through the use of heat sticks, pyrometers and other temperaturemonitoring equipment.

The welds were inspected and epoxy was injected into any remainingopenings between the corner fittings and the shelter panels and otherseams that could not be welded. After cure, the excess material wascarefully removed by sanding, for example to blend between the cornerfittings and the wall panel skins. The sanded surfaces were roughenedand cleaned with isopropyl alcohol in preparation for metal spray.

All interior fasteners (potted inserts, threaded blind rivets forexample) were installed and similarly prepared for metal spray byfilling any gaps with epoxy and sanding flush to adjacent bare metal.Any openings or penetrations in the panels (the drain plug for example)were similarly drilled, sanded to bare metal and cleaned with isopropylalcohol in preparation for metal spray. The interior surfaces wereinspected and the interior seams, corners, drain plug opening, insertsand door frame were sprayed with a bond coat of molybdenum metal spray.After spray was removed by exposure of the panels and seams to filteredcompressed air.

Within an hour, a 2 millimeter thick coat of high carbon steel wasapplied to the all interior seams and prepared surfaces of the shelter.Any after spray or other particulate contaminants from the ambient airwere removed from the surface of the shelter through the use of filteredcompressed air.

A topcoat of tin metal spray was applied to the sprayed surface of theseams and panels of the shelter within an hour of the application of thesteel coat of metal spray. After the removal of overspray from thetopcoat, the assembly of the shelter was completed in preparation fortesting by installing the shelter door assembly, door strike plates,door gaskets, drain plug and related components.

Testing of the RFI shielding capability and structural integrity of theshelter was conducted in accordance with ASTM E1851-97, with thespecific requirement that the signal effectiveness (attenuation) begreater than 20*log(f)−60 where f is the test frequency in Hz, and aminimum of 80 dB for frequencies greater than 10 MHz. Resultsdemonstrated compliance with these requirements, a significantimprovement over the current art. The structure demonstrated 2.0 poundsper square inch minimum blast overpressure capability and roof loads inexcess of 40 pounds per square foot and 660 pounds over any 2 squarefoot area. Floor loads of 65 pounds per square foot uniform load with1,000 pounds over any 2 square foot area and 125 pounds per square footpoint load. The shelter also showed a U-Factor of 0.39 BTU/hr/sq.ft./°F.

Endurance testing showed that there was no degradation of the EMI/RFIshielding capability of the shelter over time and exposure to moistureand temperature extremes as well as transportation vibrations andshocks. Attenuation capability of the shelter of approximately 80 dBpersisted through the length of the test and has remained constant overa period one and one-half years.

Accordingly, it will be seen that this invention provides a EMI/RFIshield and method that produces a shielded enclosure or shelter thatdoes not degrade over time and is resistant to weather extremes andshock and vibrations from frequent transportation.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

1. An enclosure shielded to electromagnetic and radio frequencyinterference, comprising: a top panel; a bottom panel; front, rear, leftside and right side walls joined to said top panel and to said bottompanel said front and rear walls joined to said left and right wallsconfigured to form an enclosure; electrically conductive filler disposedwithin joints between said walls and said panels; and a plurality oflayers of metal spray coating said panels, walls and filled jointsadapted to provide resistance to electromagnetic interference.
 2. Anenclosure as recited in claim 1, wherein said panels and walls compriseplanar sheets having at least one surface comprising aluminum.
 3. Anenclosure as recited in claim 1, further comprising: a coating of paintto a top layer of metal spray.